The present invention relates to a high-pressure metal vapor discharge lamp comprising a discharge tube made of a ceramic material having a transparent or translucent property.
A typical conventional high-pressure metal vapor discharge lamp will be explained with reference to FIG. 12.
FIG. 12 is a partially cross-sectional view showing a configuration of a conventional high-pressure metal vapor discharge lamp.
As shown in FIG. 12, a conventional high-pressure metal vapor discharge lamp comprises a discharge tube 51 contained in an outer tube 50, a pair of main electrodes 52a, 52b disposed inside the discharge tube 51, and an auxiliary electrode 53 disposed in the vicinity of the main electrode 52b. An external surface of the outer tube 50 is coated with a fluorocarbon resin film 50a. A mixture of Ne-gas and N.sub.2 -gas is filled in the outer tube 50. The main electrodes 52a, 52b comprises electrode rods 58a, 58b and electrode coils 61a, 61b, respectively.
The discharge tube 51 is made of quartz glass having a transparent or translucent property, and consists of a discharge part 54 for a discharge space and sealed parts 55a, 55b disposed at the both end parts of the discharge part 54, respectively. A metal halide as a luminescent material and a mixture of Ne-gas and Ar-gas or the like for a start of a lighting operation are filled into the discharge tube 51.
The sealed part 55a is mounted to one end part of the discharge part 54 together with a main electrode current supply conductor 56a for supplying a current to the electrode coil 61a by a pinch seal method. Similarly, the sealed part 55b is mounted to the other end part of the discharge part 54 together with a main electrode current supply conductor 56b and an auxiliary electrode current supply conductor 57 for supplying a current to the electrode coil 61b and the auxiliary electrode 53, respectively, by the pinch seal method.
The main electrode current supply conductor 56a is configured by integrating the electrode rod 58a which holds the main electrode 52a at one end, a molybdenum foil 59a connected to the other end of the electrode rod 58a, and an external lead wire 60a connected at one end of the molybdenum foil 59a. Similarly, the main electrode current supply conductor 56b is configured by integrating the electrode rod 58b which holds the main electrode 52b at one end, a molybdenum foil 59b connected to the other end of the electrode rod 58b, and an external lead wire 60b connected at one end of the molybdenum foil 59b. The auxiliary electrode-current supply conductor 57 is configured by integrating an electrode rod 58c which holds the auxiliary electrode 53 at one end, a molybdenum foil 59c connected to the other end of the electrode rod 58c, and an external lead wire 60c connected at one end of the molybdenum foil 59c.
In the lighting operation for the conventional high-pressure metal vapor discharge lamp, at first, an auxiliary discharge is generated between the main electrode 52b and the auxiliary electrode 53, and thereafter the auxiliary discharge is induced to a main discharge generated between the main electrodes 52a and 52b.
Particularly, a metal halide lamp, which is one of the conventional high-pressure metal vapor discharge lamps having the above-mentioned configuration, is widely used. This is because a conventional stabilizer using in a mercury lamp is available as a power source of the metal halide lamp without modification.
However, in the conventional high-pressure metal vapor discharge lamp, as explained above, the sealed parts 55a, 55b are mounted to the both end parts of the discharge part 54 by the pinch seal method, respectively. Therefore, a shape of the discharge tube 51 is not always formed into uniform size and shape, i.e., it is difficult to make the discharge tube 51 into a constant shape in mass production. Furthermore, there occurs a problem that characteristics of the lamp are varied according to different shapes of the discharge tube 51.
Moreover, in the conventional high-pressure metal vapor discharge lamp, if the shapes of the sealed parts 55a, 55b are large, a heat loss from the discharge space of the discharge tube 51 is increased. This makes it difficult to achieve a sufficient efficiency and to obtain an excellent color rendering index. Furthermore, it is necessary to seal the molybdenum foil 59b for the main electrode 52b and the molybdenum foil 59c for the auxiliary electrode 53 into the sealed part 55b in such a manner that the molybdenum foils 59b, 59c are spaced apart from each other by a predetermined gap. Therefore, it is difficult to form the sealed part 55b into a small shape.
Furthermore, in the conventional high-pressure metal vapor discharge lamp, the Ne-gas filled in the discharge tube 51 permeates through the quartz glass of the discharge tube 51. Therefore, for the purpose of preventing permeation of the Ne-gas, it is necessary to fill the mixed gas containing the Ne-gas inside the outer tube 50. However, when the mixed gas containing the Ne-gas is filled inside the outer tube 50, temperature of an external wall of the discharge tube 51 is reduced by the mixed gas. Therefore, in order to obtain and maintain a desired temperature of the external wall of the discharge tube 51 in a steady state operating condition, it is necessary to increase discharge intensity of the main discharge inside the discharge tube 51 while suppressing deterioration of a lifetime characteristic as much as possible. Further, since the deterioration of the lifetime characteristic is caused by a chemical reaction between the quartz glass of an inner wall of the discharge tube 51 and the filled metal halide therein, it is strongly desired to suppress the chemical reaction between the filled metal halide and the quartz glass which is a material of the discharge tube 51.
Any conventional high-pressure metal vapor discharge lamps attempting to reduce scattering, deviations or dispersion in the shape of the discharge tube 51, there is a high-pressure sodium lamp as disclosed in unexamined and published Japanese patent application TOKKAI (SHO) No. 51-55179, for example. In this conventional high-pressure metal vapor discharge lamp, the discharge tube is made of a ceramic material, and a metal rod equipped with the main electrode is airtightly filled with a (glass) frit to a disk-shaped ceramic disk member which is disposed instead of the sealed parts. In this manner, this conventional lamp suppresses deviations in the shape of the discharge tube and attempts to improve quality of the lamp by means of a use of the ceramic material for the discharge tube.
However, in this conventional high-pressure metal vapor discharge lamp, the metal rod equipped with the main electrode is airtightly bonded to the disk-shaped ceramic disk member by the frit. Since coefficient of expansion of the metal rod is different from that of the ceramic disk member, there occurs a problem that the filled metal and the like leak from the discharge tube through gaps generated between the metal rod and the ceramic disk member during the lighting operation. Furthermore, a chemical reaction is generated between the frit and the filled metal. In the case of the metal halide lamp which uses the metal halide as the luminescent material in particular, when the frit is used at portions where temperature becomes high during the lighting operation (e.g., contact sections between the metal rod and the ceramic disk member inside the discharge tube), undesirable chemical reaction is generated intensively. As a result, the characteristics of the lamp are deteriorated, and further the lifetime of the lamp is shortened.